A natural knee includes a distal femur, proximal tibia, and patella. The distal femur and proximal tibia comprise a tibiofemoral joint, and the distal femur and patella comprise a patellofemoral joint. Soft tissue, such as ligaments and tendons, help stabilize these joints throughout flexion of the knee. However, the natural knee can become damaged or diseased. For example, osteoarthritis can destroy articular cartilage within the tibiofemoral and/or patellofemoral joints producing painful bone-on-bone articulation and malalignment of the knee.
Surgery may be indicated to treat complications of such damage or disease. This may involve resecting portions of the distal femur, proximal tibia, and patella and replacing the resected bone with one or more prostheses. Surgery to replace the patellofemoral and tibiofemoral joints is commonly referred to as total knee arthroplasty (“TKA”) or total knee replacement.
In many circumstances, damage extends beyond the articular cartilage and bone further complicating a TKA procedure. For example, malalignment of the knee caused by osteoarthritis can impose excess stress on stabilizing soft tissue, which may result in damage and/or laxity of such structures and instability of the knee. Without resolution, such damage and/or laxity can compromise the replaced tibiofemoral and patellofemoral joints and reduce patient satisfaction.
Certain categories of prostheses have been developed to address soft tissue instability. One such category is a posterior stabilized (“PS”) prosthesis. PS prostheses are often utilized to address posterior cruciate ligament deficiency in patients who otherwise have sufficient collateral ligament stability. Many of the existing PS prostheses offer tibiofemoral rotational freedom while providing stability during flexion to prevent femoral anterior subluxation. This is often achieved by a femoral cam and tibial post mechanism.
Another TKA prosthesis category is a hinge knee prosthesis. Hinge knee prostheses are typically indicated for patients that have significant stabilizing soft tissue deficiencies, particularly significant laxity or complete removal of the collateral ligaments. Many hinge knee prostheses offer a one-degree-of-freedom hinge about which the knee flexes and extends. This prosthesis category generally provides the most stability of any of the TKA prostheses. However, this often comes at the expense of being one of the most restrictive when it comes to natural knee kinematics.
Certain hinge knee prostheses have been developed to provide additional degrees of freedom to help mimic certain aspects of natural knee kinematics. However, such prostheses generally focus on articulation between the femur and tibia and have yet to solve patellofemoral kinematics. As a result, current hinge knee prostheses generally have unnatural patellofemoral kinematics when implanted which can be painful and restrict range of motion of the knee.
Therefore, further improvements are desirable.